Conventional electric domestic hot water heaters typically comprise a tank for water storage and at least one electrical element for heating water stored in the tank. Typically, an electric hot water heater is equipped with two ohmic electrical elements for heating: one near the top of the tank, and one near the bottom of the tank.
Upper and lower thermostats are further typically located in proximity to each element. These thermostats maintain the temperature of the water in the top and bottom regions at a single pre-selected set-point. Often thermostats incorporating bi-metallic switches are used to maintain the tank temperature.
This method of heating water in a hot water tank has numerous disadvantages. From a user's point of view, this type of heater wastes energy. As the temperature of the tank is maintained near a single set-point throughout the day, energy given off by the heater to the environment is constantly replenished. Similarly, the tank may be heated at all hours of title day, even though the end user may not require a full tank of water at all hours of the day.
From an energy provider's point of view, conventional hot water heaters require power at all hours of the day. Accordingly, even though a user has no need for hot water and electric power may be at a premium to the energy provider because of peak energy demands across the entire electric power grid, the elements of a hot water heater may be consuming electrical energy.
From a consumer's perspective, a control system for the heating of an electrical hot water tank which reduces the net energy provided to the tank as compared to the energy provided by a conventional hot water heater is desirable.
From an energy provider's perspective, a control system which allows the majority of the heating of a hot water tank to be heated during times when energy is not in high demand is desirable.
Moreover, if such a system when implemented in multiple households can be used to increase power demand during off peak-energy consumption times (the "off-peak periods") and decrease power demand during peak energy consumption times (the "peak periods"), further benefits may be achieved. These benefits are realized because an electric energy provider is typically equipped to provide a certain minimum load at all times during the day. This minimum load is generated by electric generating stations whose output is difficult to vary. During the peak periods, additional electric energy may be generated by generating stations whose power output may be varied. Accordingly, the cost of producing excess energy during the peak period may be significant. Similarly, during the off-peak periods demand for electricity may fall below the minimum generated output power. Because of the nature of the generating stations used to generate this minimum power, it may difficult if not impossible to reduce the minimum power produced. Therefore, energy providers are desirous of shifting some of the load demanded by their consumers from the peak periods to off-peak periods.
It is known that the use of time shifting the heating of hot water in domestic electric hot water tanks may be used to "shift" energy demand requirements for an electric energy provider.
For example, U.S. Pat. No. 4,998,024 issued Mar. 5, 1991, naming Kirk as an inventor discloses a domestic hot water heater controller which turns a hot water heater off during peak demand times and activates the hot water heater to maintain a single net point temperature during off-peak periods.
Recognizing that maintaining the temperature at this set-point during off-peak periods may be wasteful, U.S. Pat. No. 2,238,624 issued Apr. 15, 1941 and naming Clark as inventor, discloses a hot water heater controller which does not activate the hot water heater until a critical time during an off-peak period. The critical time is calculated based on the rate at which the water in the tank may be heated and is chosen so as to provide a full tank of hot water at the beginning of the peak period.
These systems, however, do not take into account the electric energy provider's desire to evenly distribute demand during off-peak periods. U.S. Pat. No. 4,449,178 issued May 15, 1984 and naming Blau as inventor discloses a system which varies the power consumed by elements of the hot water heater by varying the duty cycle of the power provided to the elements so that the time required to fully heat a full tank water extends throughout the entire off-peak period. Thus, each hot water heater will consume a fraction of the power consumed by a conventional hot water heater, while remaining energized for a greater portion of the off-peak period.
Similarly, PCT application No. PCT/CA93/00455 naming Munroe et al. as inventors, discloses a system which distributes power consumption of a hot water heater by providing heating at intermittent, random time intervals during the off-peak period.
The known systems, however, do not provide for a flexible programming of the tank temperature at various times of the day, week, month or year. Moreover, the known systems do not provide for the direct and independent control of upper and lower tank temperature during these time intervals. Additionally, the known systems are not adapted to be simply fitted to existing tanks. Instead they require the installation of additional and temperature and control circuitry.